FIG. 4 is a diagram showing an example of an image displaying device using organic light emitting elements. FIG. 4A is a sectional view, and FIG. 4B is a top view.
Insulating layers 2 and 3 are laminated on a glass substrate 1, and an anode 4 made of a transparent conductive film which prescribes each light emitting area, is formed thereon. And further, on the entire surface thereof, an organic light emitting layer 5 and a cathode 6 are formed. Moreover on the glass substrate 1, a transistor for current driving 8 which current is supplied via a power supply line 7, a transistor 9 for ON/OFF control of the transistor for current driving 8, and a vertical electrode 10 and a horizontal electrode 11, for selecting display cells to emit light, are formed. The vertical electrode 10 and the horizontal electrode 11 are insulated by the insulating layer 2. Each transistor is protected by the insulating layer 3, and also a surface on the transistor is smoothed. In addition, the transistor for current driving 8 is connected to the anode 4 via a through hole of the insulating layer 3. Moreover, a facing seal plate 12 is provided so as to cover the entire cathode 6 for hermetically sealing the image displaying device.
In the image displaying device having such a configuration, when the transistor 9, of a display cell selected by the vertical electrode 10 and the horizontal electrode 11, turns on, the transistor for current driving 8 turns on, and a current flows from the power supply line 7 through the anode 4, the organic light emitting layer 5 and the cathode 6, thus the selected cell emits light. This light emitting state is continued until a signal for turning off is applied to the transistor 9. By thus selecting and driving each light emitting cell arranged in a matrix form by transistors, image display is conducted.
The current driven typed light emitting element such as the organic light emitting element emits light by applying current through it, so that, for maintaining the light emitting state, it is necessary to continue applying the current flow. Therefore, for active matrix driving of a current driven typed light emitting element, at least two active elements are needed, one is an element to keep applying the current flow and another is an element to control the former element. Furthermore, to keep applying the current flow, a dedicated current supply line is needed.
As shown in FIG. 4, the current driven typed light emitting element is connected to the transistor for current driving 8, and connected in between the common power supply line 7 and a common installed line (cathode). At least two transistors are required for one current driven typed light emitting element. As for the wiring, four electrodes of a data line for selecting a current driven typed light emitting element, a scanning line (the vertical electrode and the horizontal electrode), the power supply line, and a ground line are needed, resulting in a complicated structure. Especially, as the number of display pixels increases, the power supply line is required to have low resistance because higher current supply capability is required. There is a problem that the aperture ratio of pixels is decreased if the line width is increased in order to lower the resistance.
Furthermore, since the yield of the transistor circuit part and the current driven typed light emitting elements are different, there is also a problem that it is difficult to ensure a high yield and a high quality as the whole.
In some cases, a color conversion method is used in the current driven typed light emitting elements. In the color conversion method, light of a light emitting layer of a specific color is converted to light of another color by using a fluorescent dye. For example, apart of light of the blue color light emitting later is converted to green or red. In this case, the color conversion layer is formed so as to be connected to the light emitting layer. For active matrix driving of organic light emitting elements of such a color conversion method, the following two methods are conceivable.
(1) Thin film transistors are formed on a color conversion layer, and further, an organic electric field light emission layer is formed thereon.
(2) An organic electric field light emission layer is formed on thin film transistors, and further, a color conversion layer is formed thereon.
In the method of (1), the color conversion layer is formed, and thereafter the transistors are formed thereon. Therefore, the color conversion layer is required to have heat resistance of at least the process temperature 400° C. at the time of transistor fabrication, and it is extremely difficult. On the other hand, in the method of (2), the color conversion layer is formed on the light emitting layer. However, since the light emitting layer is extremely vulnerable to moisture, it is extremely difficult to form the color conversion layer directly on the light emitting layer.
As a countermeasure against them, application of, not the bottom emission method in which light is taken out from the glass substrate side as shown in FIG. 4, but the top emission method in which light is taken out from the cathode side is conceivable. This aims to implement active matrix driving of the color conversion method by individually fabricating a substrate having a light emitting layer using a transparent cathode formed thereon and a substrate having a color conversion layer formed thereon, and then putting the substrates together. In addition to a problem in reliability of the transparent cathode, there is a problem that the image quality is degraded because the light emitting layer and the color conversion layer are optically separated so that the occurrence of optical crosstalk is inevitable.